1. CLINICAL VIGNETTE OF THE MONTH-December 2015 (e- edition, vol 1:4)
Editor: Olufemi E. Idowu, Neurosurgery Division, Department of Surgery, LASUCOM/LASUTH, Ikeja, Lagos, Nigeria.
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2. Arterial Blood Gas Analysis
Ademola A. Adesina
Department of Surgery, LASUTH, Ikeja, Lagos, Nigeria.
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INTRODUCTION
Arterial blood gas (ABG) measures Acidity, Oxygen tension (PaO2), Carbon dioxide tension (PaCO2), Oxyhaemoglobin saturation and Bicarbonate concentration
*Some blood analyzers also measures- Methaemoglobin,
Carboxyhaemoglobin and Haemoglobin levels
Essential for diagnosing, monitoring a patient oxygenation status/gas exchange & acid base balance (whether patient is on mechanical ventilator or not)
*Change in H+ concentration is responded to by BICARBONATE BUFFER SYSTEM in few seconds, RESPIRATORY REGULATION in few minutes & RENAL REGULATION in hours to days
*Normal ABG result does not rule out acid base imbalance
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4. ABG COMPONENTS & NORMAL VALUES
pH- hydrogen ion concentration ( )
PaCO2- pressure of CO2 in the blood (35-45mmHg)
PaO2- pressure of O2 dissolved in the blood (72-104mmHg)
Oxygen saturation- amount of oxygen in the blood (95-100%)
HCO3- kidney ability to retain and excrete bicarbonate (22-26mEq/L)
Electrolytes- Na, K
Base Excess- assessment of metabolic acidosis/alkalosis (+ 2mEq/L)
Anion Gap- Difference in measured cations and anions (12 + 4)
*O2 Saturation measured by pulse oximeter- SpO2
*O2 Saturation measured by CO-oximeter- SaO2
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CLINICAL CORRELATE
pH- Determines presence of acidaemia or alkalaemia
PaO2-
Reflect gas exchange in the lung
From ventilation perfusion mismatch or hypoventilation
Decreases with age
*PaO2 :FiO2< 300mmHg- Acute lung injury
*PaO2:FiO2<200mmHg- Acute respiratory distress
PaCO2-
Reflect the state of alveolar ventilation
Elevated PaCO2 reflects alveolar hypoventilation
Decrease PaCO2 reflects hyperventilation
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6. OBTAINING A BLOOD SAMPLE
Explain procedure to the patient and obtain Consent
Check vital sign
Materials- Heparinized 1ml syringe, heparinized needle (23-26G)
Perform Allen’s test when using radial artery
Other Arterial sites: brachial, femoral, dorsalis pedis
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ABG ANALYZERS
i-STAT and I-STAT I Portable Clinical Analyzer
i- STAT Cartridges( i-stat G3 Cartridges)
Stored at 2- 8oC
Cartridge are used at room temperature
About 30 minutes to reach room temperarure
I-STAT Calibration verification material
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ACID BASE DISORDERS…
RESPIRATORY ACIDOSIS
pH<7.35; PaCO2> 45mmHg
CNS DEPRESSION- Head injury, Narcotic, Sedatives
Neuromuscular disease
Pulmonary disorder
Hypoventilation- Pain, chest trauma, deformity
RESPIRATORY ALKALOSIS
pH>7.45; PaCO2<35mmHg
Metabolic demand: fever, sepsis
Drugs: respiratory stimulant
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ACID BASE DISORDERS
METABOLIC ACIDOSIS
pH<7.35; HCO3<22mEq/L; Negative base excess
Renal failure
Diabetic ketoacidosis (DKA)
Salicylate poisoning
Starvation
High anion gap
Indicative of acidosis
lactic acidosis
ketoacidosis: DKA, Alcohol abuse
Toxin- methanol, ethylene glycol, ureamia, aspirin, cyanide
Renal failure
METABOLIC ALKALOSIS
pH>7.45; HCO3>26mEq/L; Positive base excess
Ingestion of antacid
Excess use of bicarbonate
Loss of acid – protacted vomiting, excess diuretics
Low anion gap
Caused by hypoalbuminemia
Nephrotic syndrome
Liver cirrhosis
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INTERPRETATION… pH
PaCO2
PaO2
Base excess
HCO3
Respiratory acidosis
Low
Increased
Normal
Normal/Increased
Respiratory alkalosis
High
Normal/Decreased
Metabolic acidosis
Reduced
Reduce
Metabolic alkalosis
Elevated
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INTERPRETATION pH
PaCO2 (mmHg)
HCO3- (mEq/L)
Respiratory Acidosis
Acute
<7.35
>45
Normal
Partly compensated
>26
Compensated
Respiratory Alkalosis
>7.45
<35
<22
Metabolic Acidosis
Metabolic Alkalosis
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CASE SCENARIO: 50-year-old man with multisystem trauma, deteriorating 5 days after admission. pH
7.25
PO2
90mmHg
PCO2
22mmHg
SO2
95%
Actual HCO3-
10.3mEq/L
Na+
139mEq/L
Cl-
100mEq/L
Interpretation
ACIDAEMIA- pH of 7.25
METABOLIC- PCO2
ANION GAP= Na – (HCO3 +Cl)
= 139-( )
= =HIGH
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FACTORS INFLUENCING ABG RESULTS *Delayed processing may yield falsely low PaO2 *Air bubbles introduced during arterial puncture can lead to falsely high PaO2 and low PaCO2. *Body temperature can affect blood gas tension
LIMITATIONS OF ABG ANALYSIS
It cannot yield specific diagnosis
The analysis does not reflect the degree to which an abnormality actually affect a patient
Cannot be used as a screening test for early pulmonary disease
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CONCLUSION
ABG results shows if the patient is acidaemic or alkalaemic and whether the cause is likely to have a respiratory or metabolic component
Useful adjunct to the assessment of patient with acute/chronic disease and in mechanically ventilated patients in ICU or intra- operatively
When combined with patient clinical features its analysis can facilitate diagnosis
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REFERENCES
Severinghaus JW, Astrup P, Murray JF. Blood Gas analysis and critical care medicine. Am j Resp Crit care med. 1998; 157:S
Burden and McQuillan. BJA. 1997; 74:479
Williams AJ.ABC of oxygen :assessing and interpreting arterial blood gases and acid base balance. BMJ. 1998; 317:1213-6
O’ Driscoll BR, Howard LS, Davison AG. BTS guideline for emergency oxtgen use in adult patients. Thorax. 2008; 63:1-68.
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